1. Field of the Invention
The invention relates to a chemical mechanical polishing slurry, more particularly to a chemical mechanical polishing slurry for polishing a copper layer without excessively or destructively polishing a barrier layer beneath the copper layer. The invention also relates to a method for preparing the chemical mechanical polishing slurry, and a chemical mechanical polishing method using the chemical mechanical polishing slurry.
2. Description of the Related Art
Chemical mechanical planarization is an essential technology in a semiconductor manufacturing process, and may affect the result of the following photolithography. The planarization requirement for a surface of a wafer is more stringent due to the fact that the diameter of the wafer is increased, the wire width of the manufacturing process is miniaturized, and the integrated density of the components is increased. A chemical mechanical polishing (hereinafter referred to as CMP) process is used for the planarization of the surface of the wafer. An advantage of the CMP process is that it can solve the problem in which the focus depth is reduced due to the lithography miniaturization during the photolithography process for fabricating wafers having highly densified integrated circuits.
A copper chemical mechanical polishing (hereinafter referred to as Cu CMP) process is a two-step polishing process for the planarization of the surface of the wafer. Copper on the surface of the wafer is removed in the first step so as to expose a barrier layer, which is primarily composed of tantalum nitride or tantalum. The barrier layer is then removed in the second step. Each of the two steps is performed using a specific polishing slurry. The polishing slurry for the first step should meet the requirements that the surface of the wafer should be as planar as possible (i.e., the dishing and erosion levels should be as low as possible), and that the polishing slurry should not cause damage to the barrier layer while achieving a high rate of copper removal.
A typical slurry for the Cu CMP process is composed of an acid or base solution, which includes an acid or base component, water, surfactant, etc., and solid polishing particles dispersed in the solution. The acid or base solution is used to react with the copper on the surface of the wafer to form a copper oxide passivated film, which can be abraded mechanically by the polishing particles.
The polishing particles may be silica, alumina, zirconia, cerium oxide, silicon carbide, titania, silicon nitride, or combinations thereof.
Taiwanese Patent No. 1235762 discloses a slurry for polishing copper-based metal comprising a silica polishing material, an oxidant, amino acid, a triazole compound, and water, wherein the weight ratio of the amino acid to the triazole compound ranges from 5 to 8, and wherein the triazole compound is selected from the group consisting of 1,2,3-triazole, 1,2,4-triazole, and the derivatives thereof. The silica polishing material is silica polishing particles commonly used in the art.
Colloidal silica particles commonly used as polishing particles are made by a wet chemical method which includes the steps of preparing a dilute solution of a water glass (e.g., sodium water glass and potassium water glass which are primarily composed of sodium silicate and potassium silicate, respectively) solution; passing the water glass solution through a cationic exchange resin to remove metallic ions such as Na+ and K+ from the water glass solution to form an aqueous active silicic acid solution; adding a basic agent to the silicic acid solution for alkalization at appropriate temperature and pH value to conduct polycondensation of active silicic acid to form particles of colloidal silica; and ultra-filtering the particles of colloidal silica to obtain a silica sol. In consideration of safety, operating convenience, and production costs, the alkalization is usually conducted using sodium hydroxide or sodium water glass.
Since the colloidal silica particles bear negative charge on the surfaces thereof, they cannot be used advantageously in an acidic polishing slurry for the first step of the Cu CMP process to remove Cu. It is well known in the art to treat the colloidal silica particles with a surface charge modifier to cationically modify the surface of the colloidal silica particles. The details of the treatment with the surface charge modifier are described in “The Chemistry of Silica”, R. K. Iler, Wiley Interscience (1979), pp. 410-441. For example, in the wet chemical method for preparing the colloidal silica particles, the salt of a metal, such as aluminum, chromium, gallium, titanium or zirconium, (e.g., sodium aluminate, or aluminium hydroxychloride can be used as the surface charge modifier. The metallic cationic ions from the surface charge modifier bonded onto the surfaces of the colloidal silica particles modify the surface charge of the colloidal silica particles so that the colloidal silica particles can be used stably in an acidic polishing slurry useable for the first step of the Cu CMP process to remove Cu. U.S. Pat. No. 5,368,833 discloses a silica sol comprising silica particles having a specific surface area within a range from 750 to 1000 m2/g and surface-modified with aluminum to a degree of from 2 to 25% substitution of silicon atoms. The sol has an S-value within the range from 8 to 45%. It is disclosed in the patent that the sols are particularly suitable for use as additives in paper making, and that the process disclosed therein is suitable for the production of sols having a comparatively broad particle size distribution.
The process for the production of silica sols disclosed in the aforesaid U.S. patent starts from an alkali water glass (preferably, sodium water glass). A water glass solution prepared from the alkali water glass is acidified and then alkalized to form silica particles, which are further processed by a cationic exchange resin treatment and a surface modification of the surfaces of the silica particles to obtain the desirable silica sols. In Column 3, Lines 9-11 of U.S. Pat. No. 5,368,833, it is described that the alkalization can be carried out with conventional alkali such as sodium, potassium or ammonium hydroxide, that it is preferred that alkalization is carried out by addition of water glass, and that potassium and sodium water glass, particularly sodium water glass, with a specific mole ratio of SiO2 to M2O, is used in the alkalization.
Since polishing particles used in a CMP slurry must have a relatively homogeneous particle distribution to achieve a superior polishing result, the sols having a comparatively broad particle size distribution as disclosed in U.S. Pat. No. 5,368,833 are not suitable for a CMP process. Moreover, such patent suggests nothing relating to a polishing slurry suitable for a CMP process.
U.S. Pat. No. 6,362,108 discloses a composition for chemical mechanical polishing comprising an acid aqueous suspension of cationized colloidal silica containing individualized colloidal silica particles and water as the suspension medium. The composition is used for polishing a layer of insulating material based on a polymer with a low dielectric constant. However, copper removal is neither taught nor suggested in this patent.
U.S. Patent Application Publication No. 2003/0094593 A1 discloses a silica and a slurry composition for chemical mechanical planarization. The silica comprises an aggregate of primary particles having an average diameter of at least 7 nanometers, and a hydroxyl content of at least 7 hydroxyl groups per nanometer squared. The slurry composition comprises the silica, and is used for the chemical mechanical planarization of a substrate such as micro-electronic substrate by removing copper, tantalum, and silicon dioxide from the substrate.
As recited in claim 17 of US Pat. Publ. No. 2003/0094593 publication, the removal of tantalum is at a rate which is equal to or greater than the removal rate of copper. Therefore, the slurry composition disclosed in this prior art is suitable for removing tantalum, rather than copper. That is, the slurry composition is suitable for the second step of a Cu CMP process for the removal of a barrier layer, rather than the first step of the Cu CMP process for the removal of copper.
Therefore, it is desirable to provide a chemical mechanical polishing slurry for the removal of copper which will not cause damage to a barrier layer beneath a copper layer while providing a high efficiency for copper removal.